Making peer reviewed research available to all.
Open access (OA) is where peer reviewed articles are made freely available online. It is not self-publishing but simply a means of making traditional peer reviewed research publications available to all would-be readers, not just those who can afford to pay for access.
It matters because open access accelerates the pace of discovery by exposing research finding to a wider audience. By harnessing the power of networks to share research findings with practitioners who can apply the new knowledge, open access also accelerates the translation of research into benefits for the public.
Consequently, most of the large research funding bodies (including the ARC and NHMRC) now insist as a condition attached to the grant that an open access version of articles arising from the funded research is made available. They see open access as a means to increase the return on the investment of taxpayer money that goes towards funding research.
The Australasian Open Access Strategy Group has more information about open access.